Immune responses of the recipient to the viral vectors used in somatic gene therapy, i.e., to the viral proteins of the vector, and/or to the transgene carried by the vector, and/or the virus-infected cells, have emerged as recurring problems in the initial application of gene therapy technology to animals and humans [Yang et al, J. Virol., 69:2004-2015 (1995) (Yang I)]. In virtually all models, including lung-directed and liver-directed gene therapy, expression of the transgene is transient and associated with the development of pathology at the site of gene transfer.
The transient nature of transgene expression from recombinant adenoviruses, for example, has been found to be due, in part, to the development of antigen specific cellular immune responses to the virus-infected cells and their subsequent elimination by the host. The collaboration of CTLs directed against newly synthesized viral proteins and viral specific T helper cells [Zabner et al, Cell, 75:207-216 (1993); Crystal et al, Nat. Genet., 8:42-51 (1994)] leads to the destruction of the virus-infected cells. These immune responses have also been noted to cause the occurrence of associated hepatitis that develops in recipients of in vivo liver-directed gene therapy within 2-3 weeks of initial treatment, and myositis in recipients of in vivo muscle-directed gene therapy.
Another antigenic target for immune mediated clearance of virus-infected cells is the product of the transgene. CTLs are an important effector in the destruction of target cells with activation occurring in some cases in the context of the transgene product, as well as the viral-synthesized proteins, both presented by MHC class I molecules [Yang I; and Zsengeller et al, Hum. Gene Thera., 6:457-467 (1995)].
Another limitation of the use of recombinant virus vectors for gene therapy has been the difficulty in obtaining detectable gene transfer upon a second administration of virus. This limitation is particularly problematic in the treatment of single gene inherited disorders or chronic diseases, such as cystic fibrosis (CF), that will require repeated therapies to obtain life-long genetic reconstitution. Diminished gene transfer following a second therapy has been demonstrated in a wide variety of animal models following intravenous or intratracheal delivery of adenovirus vectors [T. Smith et al, Gene Thera., 5:397 (1993); S. Yei et al, Gene Thera., 1:192-200 (1994); K. Kozarsky et al, J. Biol. Chem., 269:13695 (1994)]. Similar difficulties have been noted when the viral vector is other than adenovirus, i.e., retrovirus, vaccinia, and the like. In each case, resistance to repeated gene therapy was associated with the development of neutralizing anti-virus antibodies, which thwarted successful gene transfer following a second administration of virus.
Proposed solutions to these anti-viral immune responses have to date involved new designs of the virus vectors which employ fewer viral genes, as well as the pre- or co-administration of immune modulators, such as anti-CD40 ligands and other modulators identified in the art [See, e.g., Yang et al., J. Virol., 70(9) (September, 1996); International Patent Application No. WO96/12406, published May 2, 1996; and International Patent Application No. PCT/US96/03035, all incorporated herein by reference].
Nevertheless, the successful induction of specific immune tolerance to the new gene products (both virus products and transgene products) of the recombinant gene therapy vectors remains one of the most formidable challenges in gene therapy. Failure to establish immune tolerance to viral gene therapy vectors may lead to immune rejection of the transgene-expressing cells and therefore loss of transgenes.
There exists a need in the art for methods and compositions which enable the induction of specific immunologic tolerance to viral vector capsid proteins and both viral gene and transgene products being introduced and expressed in mammalian cells by general gene therapy methods.